(a) Technical Field
Embodiments of the present disclosure are directed to an optical modulation device and a driving method thereof, and more particularly, embodiments of the present disclosure are directed to an optical modulation device that includes liquid crystals, a driving method thereof, and an optical device using the same.
(b) Discussion of Related Art
Recently, optical display devices that use an optical modulation device that modulates characteristics of light have been actively developed. For example, optical display devices that can display a 3-dimensional (3D) image can divide an image with different viewpoints to be transmitted so that a viewer can perceive the image as a stereoscopic image. Optical modulation devices that can be used in an autostereoscopic 3D image display device include a lens, a prism, etc., for altering a path of light to transmit the image of the display device to a desired viewpoint.
As such, a direction of incident light can be altered by light diffraction through a phase modulation thereof.
When polarized light passes through an optical modulation device such as a phase retarder, its polarization state is changed. For example, when circularly polarized light is incident on a half-wave plate, the circularly polarized light is emitted with its rotation direction reversed. For example, when right-circularly polarized light passes through the half-wave plate, left-circularly polarized light is emitted. In this case, a phase of the circularly polarized light shifts according to an optical axis of the half-wave plate, that is, a slow axis.
Specifically, when the optical axis of a half-wave plate rotates in-plane by φ, a phase of the outputted light shifts by 2φ. Thus, when the optical axis of a half-wave plate is spatially rotated in an x-axis direction by 180° (π radians), the emitted light may have a phase modulation or shift of 360° (2π radians) in the x-axis direction. As such, when an optical modulation device induces a phase shift from 0 to 2π as a function of a position, a diffraction grating or prism that can alter or diffract the transmitted light can be implemented.
Liquid crystals may be used to adjust the optical axis of an optical modulation device, such as a half-wave plate, as a function of position thereof. In an optical modulation device implemented as a phase retarder that uses liquid crystals, long axes of the liquid crystal molecules, which are aligned by applying an electric field to a liquid crystal layer, may rotate to change the phase modulation as a function of position. The phase of the light emitted after passing through an optical modulation device may be determined by the directions of the longer axes of the aligned liquid crystal molecules, that is, the azimuthal angle.
To implement a prism, diffraction grating, lens, etc., using an optical modulation device that uses the liquid crystals to continuously modulate phase, the liquid crystal molecules should be aligned so that directions of their long axes can continuously change as a function of position. For emitted light to have a phase profile that changes from 0 to 2π as a function of position, a half-wave plate should have an optical axis that changes from 0 to π. For this purpose, liquid crystals need to be aligned in different directions as a function of position with respect to a substrate adjacent to the liquid crystal layer.